The history of BMI and why we still use it

Slate.com has one of the best histories of BMI I’ve ever seen and why, despite it’s demonstrated ineffectiveness, doctors nonetheless persist in using it. The story, while succinct, is still two pages long, so I’ll paraphrase the highlights here, supplemented with some of my own commentary:

Belgian polymath Adolphe Quetelet devised what we now know as the BMI equation in 1832 as a way to define the “normal man.” He never intended for the equation (weight equals height squared) to be used to determine body fat — his project was intended to describe the standard proportions of the human build. The equation was largely ignored by the medical community even though insurance companies began using somewhat vague comparisons of height and weight among policyholders beginning in the early twentieth century. Slate writer Jeremy Singer-Vine doesn’t go in much detail about these tables, but I’ve written on their history here.

Medical researchers searched for an accurate, uniform way to measure fatness for decades when in 1972, physiology professor and obesity researcher Ancel Keys published his “Indices of Relative Weight and Obesity,” a landmark study of more than 7,400 men in five countries. Keys considered the various height-weight formulas in existance and found Quetelet’s equation to be the best marker of body-fat percentage. He renamed this number the body mass index or BMI. As Singer-Vine reports, the new number caught on among researchers who had previously relied on slower and more expensive measures of body fat or on the broad and ambiguous categories defined by the insurance companies. The number also sowed the seeds for the later and continuing bombardment of anti-obesity research. “The cheap and easy BMI test allowed [researchers] to plan and execute ambitious new studies involving hundreds of thousands of participants and to go back through troves of historical height and weight data and estimate levels of obesity in previous decades,” writes Singer-Vine.

At first BMI was used by epidemiologists in studies of population health, but was quickly adopted by doctors who wanted a quick and easy way to measure body fat in their patients. By 1985, the National Institutes of Health began defining obesity according to body mass index. At first, the thresholds were established at 27.8 for men and 27.3 for women. Then in 1998, the NIH consolidated the threshold for men and women — even though the relationship between BMI and body fat is different by sex — and added the category of overweight. The new, drastically lowered thresholds were now 25 for overweight and 30 for obesity. It’s worth adding here that many who were on the “independent” board making the recommendations for the new lower cutoffs had ties to the commercial weight-loss industry and stood to profit financially should more people be considered overweight and obese.

Here’s the kicker: Like Quetelet, Keys never intended for BMI to be used in this way. In fact, his original paper warned against using BMI for individual diagnoses, since the equation ignores variables like a person’s age or gender, and I would also add, also their ethnicity, frame size and muscle mass ratio. Writes Singer-Vine:

It’s one thing to estimate the average percent body fat for large groups with diverse builds, Keys argued, but quite another to slap a number and label on someone without regard for these factors… Now Keys’ misgivings are gaining traction across the world of medicine: BMI simply doesn’t work when it comes to individual measurements.

No matter how attentive they might be, health professionals have increasingly used body mass index to justify lifestyle recommendations for their patients. And online BMI calculators—there’s even one hosted by the NIH—invite people to diagnose themselves without any medical supervision whatsoever. Faulty readings could promote a negative self-image among healthy people and lead them to pursue unnecessary diets. Or the opposite problem: People with a little too much body fat might be lulled into a false sense of complacency by a misleading BMI.

Singer-Vine points out (as I’ve noted before) that waist-to-hip ratios are a much more accurate way of determining the kinds of body fat that might actually pose health risks. And WHR, as it’s called, is just as easy and quick to record as BMI yet few doctors have made the switch. Why? WHR require slightly more time and training than it takes to record BMI and they don’t come with any official cutoffs that can be used to make easy assessments. “The body mass index is cheap and easy, and it has the incumbent advantage,” concludes Singer-Vine. “In short, BMI is here to stay—despite, but also because of, its flaws.”